Method and apparatus for determining signal usability

ABSTRACT

A receiver that receives a stream of information symbols may detect the usability of the stream of information symbols received as follows. Upon receiving the stream of information symbols, the receiver separates the stream of information symbols into a desired portion and an undesired portion wherein the desired portion is the originally transmitted stream of information symbols and the undesired portion is interference and noise. From the undesired portion and the desired portion, the usability can be determined based on a ratio of the two portions.

FIELD OF THE INVENTION

This invention relates generally to communication systems and, inparticular, to a communication system with geographic reuse ofcommunication resources.

BACKGROUND OF THE INVENTION

Communication systems that geographically reuse communication resourcesare known. These systems allocate a predetermined set of communicationresources in one geographic area and reuse the same set of communicationresources in one or more other geographic areas. This reuse techniqueimproves communication capacity by minimizing the number ofcommunication resources necessary to provide communication service in alarge geographic area comprised of several smaller geographic areas. Asis also known, a communication resource may be a frequency carrier, apair of frequency carriers, a time slot in a time division multiplex(TDM) time frame, or any radio frequency (RF) transmission medium.

Two of the most common communication systems which geographically reusecommunication resources are cellular communication systems and trunkedmobile communication systems. In both communication systems, allocationof a communication resource begins when a communication unit requestscommunication service. Based on resource availability and signalusability, a resource controller assigns the communication resource tothe communication unit. A communication, such as a conversation or afacsimile transmission, occurs on the communication resource between thecommunication unit and another communication unit or between thecommunication unit and a subscriber to a public service telephonenetwork. The communication continues until completion or an interruptionin service occurs. Upon conclusion of the communication, the resourcecontroller retrieves the communication resource; thus making thecommunication resource available for another communication.

An important parameter in identifying an acceptable communicationresource is signal usability. In a wireless communication system, thecommunication resources are typically RF channels which occupypredetermined bandwidths. When information signals are transmitted onthe RF channels, undesired channel effects, such as fading andinterference, alter the information signals during transmission. Thus,the information signals received by a receiver in the communicationunit, or a base station, are corrupted by the undesired channel effects.By ascertaining an indication of the corruption on availablecommunication resources, the least corrupted communication resource maybe selected for the communication. This indication of corruption isknown as signal usability.

In geographic reuse communication systems, signal usability is typicallylimited by the quantity of co-channel interference present on the RFchannel. Co-channel interference occurs when receivers receive unwantedinformation signals from neighboring communication units, or basestations, transmitting on the same channel as the desired RF channel.Thus, the signal usability decreases as the co-channel interferenceincreases.

Received signal strength indication (RSSI) and bit error rate (BER) aretwo common methods of estimating signal usability. In an RSSI estimate,the receiver measures the level of a received signal on the desired RFchannel. This measurement provides a summation of signal levels (i.e.C+I+N) including the desired information signal (C), the co-channelinterference (I), and the noise (N) on the desired RF channel. Althoughthis technique accurately estimates the level of the received signal, itcannot distinguish between the desired information signal and signalsdue to co-channel interference. Thus, an acceptable RSSI measurement mayprovide unacceptable signal usability due to a high level of co-channelinterference. Alternatively, BER measurements provide accurate estimatesof signal usability, but in geographic areas where error rates are low,multiple measurements and excessive averaging times may be required toobtain the accurate estimates. Measurement periods as long as ten tofifty seconds may be necessary to obtain accurate BER data.

As briefly mentioned above, fading is an additional undesired channeleffect that may alter the transmitted information signal. Fading occursdue to multiple reflections of the desired information signal duringtransmission over the RF channel. These reflections are typically causedby unintentional reflecting of the transmitted information signal fromobstacles in its path, such as buildings and mountains, and may producemultiple modified replications of the transmitted information signal.,each introducing various amplitude and phase alterations of the originalsignal in each new signal path. All of the transmitted informationsignal replicas form a composite information signal at the input to areceiver. The signal usability of the composite signal is dependant onthe type of fading.

Two types of fading generally encountered are flat fading and frequencyselective fading. In a digital transmission, flat fading occurs when themaximum differential time delay between each new signal path is muchless than a symbol period. As is known, methods for estimating the flatfading of an RF channel exist and are used to minimize the degradationin signal usability attributed to flat fading. Frequency selectivefading occurs when the maximum differential time delay between each newsignal path is comparable to or greater than the symbol period.Frequency selective fading may also degrade signal usability. Recenttechnological advances have allowed signal quality to be estimated inthe presence of frequency selective fading. For a detailed discussion ofa method for measuring signal quality that accounts for frequencyselective fading refer to U.S. Pat. No. 5,170,413, entitled "ControlStrategy For Reuse System Assignments And Handoff", assigned to MotorolaInc. While this technology provides many advantages, it does not addressthe technological concern of estimating signal usability based on ameasured approximation of co-channel interference and noise.

Therefore, a need exists for a method and apparatus that determinesignal usability based on a determination of RF channel interference.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates constellation depictions of transmitted and receivedaltered information symbols in accordance with the present invention.

FIG. 2 illustrates a block diagram depiction of a communication systemthat includes a receiver in accordance with the present invention.

FIG. 3 illustrates an alternative block diagram depiction of acommunication system that includes a receiver in accordance with thepresent invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

Generally, the present invention provides a method and apparatus fordetermining a level of usability of a received signal. The receivedsignal comprises a summation of a desired portion and an undesiredportion. Typically, the desired portion includes an original transmittedsignal, modified in amplitude and phase, and the undesired portionincludes noise and interference. In the present invention, a receiverextracts a representation of the desired portion from the receivedsignal and utilizes it to obtain a representation of the receivedsignal's undesired portion. The receiver then calculates the averagepower contained in each portion and forms the ratio of these two powers.This ratio, which is commonly referred to as the carrier to interferenceplus noise ratio, or C/(I+N), provides an indicia of the usability ofthe received signal.

The present invention can be more fully described with reference toFIGS. 1-3. FIG. 1 illustrates a transmitted information symbol pattern100 and a received altered information symbol pattern 101. Thetwo-dimensional symbol patterns illustrated in FIG. 1 are typical of adigital transmission system using 16-ary quadrature amplitude modulation(QAM), however, symbol patterns corresponding to alternate digitalmodulation schemes, such as quaternary phase shift keying (QPSK) anddifferential QPSK, are also commonly used in the art.

In a digital transmission system, the transmitter emits a stream ofinformation symbols, wherein each information symbol is a particularvalue chosen from a discrete set of possible values contained in thetransmitted information symbol pattern 100. For example, at a particularsymbol time, the transmitter may transmit the information symbol 103indicated in the transmitted information symbol pattern 100. Thetransmitted stream of information symbols propagates from thetransmitter to a receiver over a communication resource, such as an RFtransmission channel. The information symbol stream received by thereceiver is typically altered due to undesired effects encounteredduring transmission over the transmission channel.

The transmitted information symbol 103 is altered in two ways. First,the communication resource modifies the symbol in amplitude and phase,in effect rotating and scaling the transmitted information symbolpattern 100 to produce the received altered information symbol pattern101. The exact amount of rotation and scaling introduced by thecommunication resource generally changes in a random manner as afunction of time due to the effects of Rayleigh, or flat, fading.Secondly, the communication resource adds noise and interference to thetransmitted information symbol 103, such that a received informationsymbol is displaced by a certain random amount. In a multiple sitecommunication system that utilizes geographic reuse of its communicationresources, the interference arises from the use of the communicationresource by other transmitters in the system. The noise results fromvarious sources such as thermal noise and environmental noise. Themultiple site communication system is typically designed to insure thatthe noise and interference introduced by the transmission channelremains small compared to the transmitted information symbol 103, thusin normal operating conditions, the received information symbol lies,with high probability, in a region 102 near its correspondingtransmitted information symbol.

As a result of the alterations introduced by the communication resource,the received information symbol stream comprises desired and undesiredportions. The desired portion, which includes the information symbolstream originally transmitted by the transmitter, is the portion that isrotated and scaled due to fading. The undesired portion is theinterference and noise added to the transmitted symbol stream by thecommunication resource.

FIG. 2 illustrates a communication system 200 that includes atransmitter 20 1, an RF channel 202, a receiver 203, and noise andinterference 204. The transmitter 201 may be included in a base stationor communication unit, such as a two way radio or radiotelephone.Similarly, the receiver 203 may also be included in a communication unitor a base station. As is known, communication units transceiveinformation, via the RF channel 202 or a communication resource, withthe base station. The manner in which these communications areestablished is known and not germane to this discussion.

Signals, such as the stream of information symbols, are transmitted bythe transmitter 201 over the RF channel 202 or a communication resource,where they encounter noise and interference 204, and are received by thereceiver 203. The received signals, which include a representation ofthe transmitted signals summed with the noise and interference 204,enter an apparatus 210 within the receiver 203 that comprises a desiredportion determiner 205, an undesired portion determiner 206, and asignal usability determiner 209. The desired portion determiner 205accepts the received signals and determines the desired portion fromthem. In this embodiment, the desired portion includes an estimate ofthe representation of the transmitted signals and is obtained byconverting the received signal from analog to digital form and applyingdigital signal processing. The undesired portion determiner 206 isoperably coupled to the desired portion determiner 205 and accepts thereceived signals and the desired portion from which it determines anundesired portion of the received signals. The undesired portiondeterminer 206 may comprise a subtractor that subtracts the desiredportion from the received signals to determine the undesired portion.The signal usability determiner 209 receives the desired and undesiredportions from the desired portion determiner 205 and the undesiredportion determiner 206, respectively, and determines an indicia ofusability of the received signals based on them. As briefly mentionedabove, the indicia of usability generally includes evaluating a ratio ofthe desired portion to the undesired portion.

The apparatus 210 may also include an undesired portion averager 207 anda desired portion averager 208 which accept the undesired and desiredportions, respectively, and produce corresponding averages of them. Thesignal usability determiner 209 utilizes the averages produced by theundesired portion averager 207 and the desired portion averager 208 todetermine the indicia of usability. Typically, the averages produced bythe undesired and desired portion averagers 207 and 208 are averagesignal powers determined by averaging the squared magnitude of thecomplex undesired and desired portions, respectively; thus, the indiciaof usability provided by the signal usability determiner 209approximates the aforementioned carrier to interference plus noiseratio. The apparatus 210 within the receiver 203 may be a digital signalprocessing (DSP) unit that includes software routines which function asthe undesired and desired portion determiners 205 and 206, the undesiredand desired portion averagers 207 and 208, and the signal usabilitydeterminer 209.

FIG. 3 illustrates a communication system 300 that includes thetransmitter 201, the RF channel 202, and a receiver 301. The transmitter201 transmits a stream of unaltered information symbols over the RFchannel 202 to the receiver 301. In the RF channel 202, the unalteredinformation symbol stream encounters multiplicative effects of fading302 and additive effects of noise and interference 204 resulting in astream of altered information symbols at the input to the receiver 301.For example, each information symbol of the unaltered information symbolstream may be represented by a discrete value D. The effects of fading302 in the RF channel 202 are modeled by multiplying each unalteredinformation symbol by a fading signal h and the effects of noise andinterference 204 are represented by adding an interference signal I toeach faded information symbol. Upon completing their transmissionthrough the RF channel 202, the composite stream of information symbolspresent at the receiver's input forms the altered stream of informationsymbols. Each symbol of the altered stream may be mathematically denotedby Dh+I. Therefore, each symbol of the altered stream comprises the sumof a desired portion and an undesired portion, where the desired portionis denoted Dh and the undesired portion is denoted I.

The receiver 301, which comprises a signal receiver 303, a symboldeterminer 304, a fading signal determiner 305, a desired portiondeterminer 306, the undesired portion determiner 206, and the signalusability determiner 209, is used to process the altered stream andprovide a determination of altered stream usability. The altered streamof information symbols enters the receiver 301 through an antenna andpropagates to an input of the signal receiver 303. The signal receiver306 amplifies, filters, and converts the altered stream from analog todigital form. The digital representation of the altered stream isprovided to inputs of the symbol determiner 304 and the fading signaldeterminer 305 wherein a representation of each unaltered informationsymbol D and a representation of the fading signal h are respectivelydetermined. For a detailed discussion on the technique of determiningthe representations of the fading signal and each unaltered informationsymbol refer to co-pending U.S. patent application Ser. No. 07/783,289,entitled "Communication Signal Having A Time Domain Pilot Component",assigned to Motorola Inc.

The representation of each unaltered information symbol D and therepresentation of the fading signal h are submitted to the desiredportion determiner 306 where a desired portion of the altered stream isdetermined based on its two input representations. For example, thedesired portion determiner 306 may be a digital multiplier that providesan output signal Dh. The undesired portion determiner 206 accepts inputsfrom the desired portion determiner 306 and the signal receiver 303 andmanipulates them to produce an undesired portion of the altered stream.As previously discussed with reference to FIG. 2, the undesired portiondeterminer 206 may comprise a subtractor that subtracts the desiredportion Dh from each symbol of the altered stream Dh+I to produce theundesired portion Dh+I-Dh. Provided the representations of eachunaltered information symbol D and the fading signal h are accurate, theundesired portion approximates the interference signal I. The outputs ofthe fading signal determiner 305 and the undesired portion determiner206 are provided to the signal usability determiner 209 fordetermination of an indicia of usability of the altered stream. Theindicia of usability is determined by evaluating the ratio of the fadingsignal representation to the undesired portion, i.e. h/I.

The receiver 301 may also include the undesired portion averager 207 andthe desired portion averager 208 which accept the undesired portion andthe representation of the fading signal, respectively, and produceaverages of their respective inputs. The average produced by the desiredportion averager 208 may include scaling an average of the fading signalby a predetermined average of each unaltered information symbol. Theaveraged outputs of the undesired portion averager 207 and the desiredportion averager 208 are delivered to the signal usability determiner209 for determination of the received signal usability. As previouslydiscussed with reference to FIG. 2, the averages produced by theundesired and desired portion averagers 207 and 208 are typicallyaverage signal powers; thus, the indicia of usability provided by thesignal usability determiner 209 approximates the carrier to interferenceplus noise ratio. With this technique, the indicia of signal usability(C/(I+N)) can be determined within five seconds.

The present invention provides a method and apparatus for determining anindication of received signal usability. With the present invention, areceiver in a multiple site frequency reuse communication system canrapidly and accurately evaluate the usability of a received signal,which is an important procedure for system operations such as channelassignment and handoff. The method of the present invention provides amuch better indication of received signal usability compared to areceived signal strength measurement since it differentiates betweendesired and undesired portions of the received signal, whereas thereceived signal strength measurement does not. Further, the method ofthe present invention provides a more rapid, but equivalently accurate,indicia of received signal usability compared to a bit error ratemeasurement, especially in geographic areas of low error rates, whichresults in improved operation of the multiple site communication system.

We claim:
 1. In a communication system that includes a plurality ofcommunication units, a limited number of communication resources thatare transceived by a plurality of base stations, and a plurality ofsites, wherein the limited number of communication resources aredistributed and reused throughout the plurality of sites, wherein eachof the plurality of communication units includes a receiver and atransmitter, and wherein each of the plurality of base stations includesa receiver and a transmitter, a method for determining usability ofreceived signals, the method comprises the steps of:a) transmitting, bya transmitter of either a first communication unit or a first basestation, a stream of information symbols to produce a symbol stream,wherein each information symbol of the symbol stream is one of apredetermined set of discrete values; b) receiving, by at least onereceiver of either a second communication unit or a second base station,a stream of altered information symbols to produce an altered stream,wherein each altered information symbol of the altered streamcorresponds to one of the information symbols of the symbol streammultiplied by a fading signal and summed with an interference signal; c)determining, by the at least one receiver, a representation of thefading signal from the altered stream to produce a determined fadingsignal; d) determining, by the at least one receiver, a representationof the information symbols in the symbol stream from the altered streamto produce determined information symbols; e) determining, by the atleast one receiver, a desired portion of the altered stream based on thedetermined information symbols and the determined fading signal; f)determining, by the at least one receiver, an undesired portion of thealtered stream based on the desired portion of the altered stream andthe altered stream; and g) providing, by the at least one receiver, anindicia of usability of the altered information symbols based on thedesired portion of the altered stream and the undesired portion of thealtered stream.
 2. In the method of claim 1, the determination of step(e) comprises multiplying the determined fading signal with eachdetermined information symbol to obtain the desired portion of thealtered stream.
 3. In the method of claim 2, the determination of step(f) comprises subtracting the desired portion of the altered stream fromthe altered stream to determine the undesired portion of the alteredstream.
 4. In the method of claim 1, the providing the indicia ofusability of step (g) comprises averaging a norm of the desired portionof the altered stream to produce a desired average; averaging a norm ofthe undesired portion of the altered stream to produce an undesiredaverage; and determining a ratio of the desired average to the undesiredaverage.
 5. A receiver, wherein the receiver is included in either acommunication unit or a base station in a communication system, thereceiver comprises:a signal receiver that receives a stream of alteredinformation symbols, wherein each altered information symbol in thestream of altered information symbols is a resultant of an unalteredinformation symbol multiplied by a fading signal and summed with aninterference signal; a fading signal determiner, operably coupled to thesignal receiver, wherein the fading signal determiner determines arepresentation of the fading signal to produce a determined fadingsignal; a symbol determiner, operably coupled to the signal receiver,wherein the symbol determiner determines a representation of each of theunaltered information symbols to produce determined information symbols;a desired portion determiner, operably coupled to the fading signaldeterminer and the symbol determiner, wherein the desired portiondeterminer determines a desired portion of the stream of alteredinformation symbols based on the determined fading signal and thedetermined information symbols; an undesired portion determiner,operably coupled to the desired portion determiner and the signalreceiver, wherein the undesired portion determiner determines anundesired portion of the stream of altered information symbols based onthe desired portion of the stream of altered information symbols and thestream of altered information symbols; and a signal usabilitydeterminer, operably coupled to the desired portion determiner and theundesired portion determiner, wherein the signal usability determinerdetermines an indicia of usability of the altered information symbolsbased on the desired portion of the stream of altered informationsymbols and the undesired portion of the stream of altered informationsymbols.
 6. In the receiver of claim 5, the undesired portion determinercomprises a subtractor that subtracts the desired portion of the streamof altered information symbols from the stream of altered informationsymbols to determine the undesired portion of the stream of alteredinformation symbols.
 7. The receiver of claim 5 further comprises adesired portion averager, operably coupled to the desired portiondeterminer and the signal usability determiner, and an undesired portionaverager, operably coupled to the undesired portion determiner and thesignal usability determiner, wherein the signal usability determineruses averages produced by the desired portion averager and the undesiredaverage portion to produce a ratio of the desired portion of the streamof the altered information symbols to the undesired portion of thestream of the altered information symbols to determine the indicia ofusability of the altered information symbols.